数据库三范式
- 1.保持每列字段的原子性
保存用户的地址不用一个字段address来保存，而是用几个字段组成，比如：country，province，city，location
- 2.在第一范式基础上，确保表中的每列都和主键相关（保持表的纯粹，即保持表的原子性）
一个表中只能保存一类数据，不可以把多类数据保存在同一张数据库表中，比如，学生信息表里不应该有成绩字段
- 3.在第二范式基础上，每列字段都和主键直接关系，而不是间接关系
字段之间不能相互依赖
jdbc连接Url格式：
jdbc:mysql://[host:port]/[database]
jdbc:mysql://localhost:3306/DBTest
jdbc驱动下载
https://dev.mysql.com/downloads/connector/j/
连接代码
jdbc.properties文件
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/DBTest
user=masaike
pwd=马赛克
JDBCUtil Class
/**
* 描述:JDBCUtil.
*
*
* @author yanwenqiang.
* @date 2018/11/12
*/
public class JDBCUtil {
private String driver;
private String url;
private String user;
private String password;
private Connection connection;
public JDBCUtil() {
Properties prop = new Properties();
//加载配置文件
try {
prop.load(this.getClass().getResourceAsStream("jdbc.properties"));
} catch (IOException e) {
e.printStackTrace();
}
this.driver = prop.getProperty("driver");
this.url = prop.getProperty("url");
this.user = prop.getProperty("user");
this.password = prop.getProperty("pwd");
try {
long start = System.currentTimeMillis();
Class.forName(driver);
long end = System.currentTimeMillis();
System.out.println("反射耗时：" + (end - start));
} catch (Exception e) {
e.printStackTrace();
}
}
public Connection getConnection() throws SQLException {
connection = DriverManager.getConnection(url, user, password);
return connection;
}
public void closeConn() throws SQLException {
if (this.connection != null) {
this.connection.close();
}
}
}
为什么不直接new一个Driver？源代码Driver中静态代码块里就在注册一个Driver，如果采用传统new方式，会两次new Driver.
上源码：
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
package com.mysql.cj.jdbc;
import java.sql.DriverManager;
import java.sql.SQLException;
public class Driver extends NonRegisteringDriver implements java.sql.Driver {
public Driver() throws SQLException {
}
static {
try {
DriverManager.registerDriver(new Driver());
} catch (SQLException var1) {
throw new RuntimeException("Can't register driver!");
}
}
}
Command Line
mysql -u root -p 登录
show full processlist; 显示完整的进程列表
show processlist; 显示简单进程列表
sql语句
left outer join (outer可省略)
理解: 左连接. 以左边表为基准，右边表数据可能为空
格式: select * from [左表] [左表别名] left join [右表] [右表别名] on [左表别名].[左表关联字段] = [右表别名].[右表关联字段]
例子: select * from student s left join stuSubject b on s.id = b.sid;
right outer join (outer可省略)
与left outer join 相反
inner join
理解: 内连接. 两张关联表都有数据的才会被查出
格式: select * from [左表] [左表别名] inner join [右表] [右表别名] on [左表别名].[左表关联字段] = [右表别名].[右表关联字段]
例子: select * from student s inner join stuSubject b on s.id = b.sid;
group by
理解: 从英文上理解 就是分组的意思。需要和 聚合函数（例如：max（），count（），avg（）等）
配合使用，使用时至少有一个分组标识字段（例如某一列的列名）
格式: select [聚合函数],[分组字段] from [表名] group by [分组字段];
例子: select count(id),subject_name from stuSubject group by subject_name;
order by
理解: 从英文上理解就是排序。一般后面跟上某一列的列名，然后查询到的数据会以该列的
大小顺序进行排序显示，默认的是ASC（从小到大排列），desc（大到小排列）
格式: select * from [表名] order by [要排序的字段];
例子: select * from student order by age;
where
理解: 对查询到的内容进行筛选，where后面跟上限定条件，where使用在分组和排序之后.
简单理解:用在非分组限定条件
例子: select * from student where sname='张三';
having
理解: 作用是筛选满足条件的分组，即在分组之后过滤数据，条件中包含聚组函数，
使用having条件显示特定的组，也可以使用多个分组标准进行分组.
简单理解:用在分组限定条件
例子: select min(id),subject_name from stuSubject group by subject_name having subject_name!='语文';
mysql语句示例
create database DBTest;
use DBTest;
create table student(
id int primary key,
sname nvarchar(50) not null,
age int,
class nvarchar(50)
);
create table stuSubject(
id int primary key,
sid int,
subject_name nvarchar(100) not null,
foreign key(sid) references student(id) on delete cascade on update cascade
);
-- 插数据
use DBTest;
insert student values (1,'张三',20,'一年级');
insert student values (2,'李四',23,'一年级');
insert student values (3,'王五',25,'一年级');
insert stuSubject values (1,1,'语文');
insert stuSubject values (2,1,'数学');
insert stuSubject values (3,1,'化学');
insert stuSubject values (4,2,'语文');
insert stuSubject values (5,2,'物理');
insert stuSubject values (6,3,'语文');
insert stuSubject values (7,3,'历史');
-- 简单查询
select * from student;
select * from stuSubject;
select * from student where sname='张三';
select * from student order by age;
-- 查询按照学科分组后，每组最大的id
select max(id),subject_name from stuSubject group by subject_name;
select min(id),subject_name from stuSubject group by subject_name having subject_name!='语文';
-- 查询每个学科有多少
select count(id),subject_name from stuSubject group by subject_name;
-- select min(id),sid,subject_name from stuSubject group by subject_name order by sid;
-- 全表左连接
select * from student s left join stuSubject b on s.id = b.sid;
-- 选择字段左连接
select s.*,b.sid,b.subject_name from student s left join stuSubject b on s.id = b.sid;
-- 选择字段左连接 & 子查询
select * from
(select s.*,b.sid,b.subject_name from student s left join stuSubject b on s.id = b.sid)
as m where id=1;
-- 测试外键关联字段的级联更新
select * from student where id=3;
select * from stuSubject where sid=3;
update student set id=3 where id=6;
select * from student where id=6;
select * from stuSubject where sid=6;